Why wasn't the PSP vs. DS a repeat of the PS1 vs N64?

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Another thing was that PSP lacked killer apps compared to the DS.
Outside of Monster Hunter in Japan and maybe GTA in the West, it had no answer to DS megatons like NSMB, Brain Age, Nintendogs, Mario Kart, Pokémon, Animal Crossing, etc.

Only 1 PSP game sold over 5 million copies, while the DS had 15 over this threshold.